18.2.py

from math import prod
with open('input/18.txt') as f:
  lines = [line.strip().replace(' ','') for line in f.readlines()]
  equations = [[x for x in line] for line in lines]

def get_result(x, y, op):
  if op == '+':
    return int(x) + int(y)
  if op == '*':
    return int(x) * int(y)

def get_subterms(e):
  # get the enclosed list
  subterms = []
  level = -1
  for y in e:
    if y == '(':
      level -= 1
    elif y == ')':
      level += 1
    if level == 0:
      break
    subterms.append(y)
  return subterms

def group(e):
  op_stack = []
  term_stack = []
  skip = 0
  for i, term in enumerate(e):
    if skip:
      skip -= 1
      continue

    if term in ['+','*']:
      op_stack.append(term)
    elif term == '(':
      subterms = get_subterms(e[i+1:])
      term_stack.append(group(subterms))
      skip = len(subterms)
    elif term == ')':
      pass
    else:
      term_stack.append(term)
  
  # do additions
  for i, op in filter(lambda x: x[1]=='+', enumerate(op_stack, 1)):
    if '(' in term_stack[i-1]:
      left = group(term_stack[i-1][1:-1])
    else:
      left = int(term_stack[i-1])
    if '(' in term_stack[i]:
      right = group(term_stack[i][1:-1])
    else:
      right = int(term_stack[i])
    term_stack[i-1] = '.' # mark for removal
    term_stack[i] = str(get_result(left, right, op))

  # remove spare elements
  term_stack = [term for term in term_stack if term != '.']
  op_stack = [op for op in op_stack if op != '+']
  
  # do multiplications
  for i, op in enumerate(op_stack, 1):
    if '(' in term_stack[i-1]:
      left = group(term_stack[i-1][1:-1])
    else:
      left = int(term_stack[i-1])
    if '(' in term_stack[i]:
      right = group(term_stack[i][1:-1])
    else:
      right = int(term_stack[i])
    term_stack[i-1] = '.' # mark for removal
    term_stack[i] = str(get_result(left, right, op))

  # remove spare elements
  term_stack = [term for term in term_stack if term != '.']
  op_stack = [op for op in op_stack if op != '+']

  return term_stack[-1]

example_results = [
  '(1+2)*(3+4)*(5+6)',
  '2*(3+(4*5))',
  '(5+(8*(3+9+3)*4*3))',
  '5*9*(7*3*(3+9)*(3+((8+6)*4)))',
  '(((((2+4)*9)*(((6+9)*(8+6))+6))+2)+4)*2',
]
print('Part Two:', sum([int(group(e)) for e in equations]))